1. Field of The Invention
This invention relates generally to cross-flow filters comprising a multiplicity of stacked filter plates, of a type wherein filter elements are disposed between adjacently paired stacked plates.
2. Description of the Related Art
Stacked plate cross-flow filters are utilized in a variety of solids-liquid separation operations, including the dewatering of solids-liquid suspensions such as aqueous biomass suspensions, the desalting of proteins, and the removal of secreted metabolites from cellular cultures. In such systems, the stacked plates making up the cross-flow filter are typically mounted in a unitary frame structure whereby the respective plates are retained in alignment, in a so-called "plate and frame" construction. A unitary liquid feed conduit provided with openings at spaced intervals along its length and extending through the stacked plates is typically employed as a feed means from which influent solids-containing liquid is introduced into the flow channels defined between adjacent plates in the stacked plate assembly. The flow channels in the plate and frame filter contain filter elements, such as disposable filter paper sheets, with which the solids-containing liquid is contacted and through which solids-depleted liquid passes. A unitary liquid withdrawal conduit featuring openings at spaced intervals along its length extends through the stacked plates in liquid flow communication with the respective flow channels of the stacked plate assembly and conveys solids-depleted liquid out of the filter system.
As filtration proceeds, the filtered solids build up in the flow channels of the filter, on the "feel liquid sides", i.e., active filtration surfaces, of the filter sheets. The filter is then periodically backwashed, or alternatively, it may be fully shut down after a predetermined level of solids has accumulated in the flow channels on the filtration surfaces of the filter sheet elements, following which the system is drained of liquid, and the filter sheets replaced as necessary.
In one type of presently marketed stacked filter system, commercially available from Millipore Corporation (Bedford, Mass.) as the Prostak.RTM. cross-flow filter, the adjacent filter plates define a flow channel. Solids-containing influent liquid is fed at one side of the plate from a central location into a transversely extending feed distribution conduit, which is provided with openings at spaced apart intervals along the length of the conduit for egress of the solids-containing liquid. At the opposite side of the adjacent plates, the flow channel is similarly constructed with a liquid collection conduit having openings along its length to collect the solids-depleted liquid and discharge same from a central outlet communicating with the collection conduit.
A major problem which has been encountered in cross-flow filters of the above-described type is that the liquid flow distribution, as for example reflected by the volumetric liquid flow rate or liquid superficial velocity, is highly non-uniform in the transverse directions of the flow channel. Such maldistribution of the solids-containing liquid is a result of the fact that the influent liquid is introduced into the feed distribution conduit at a central location. Due to the pressure drop in the transverse direction, from the medial inlet port out to the extremities of the feed distribution conduit, the local longitudinal flow (cross-flow of liquid from the inlet side to the outlet side of the stacked plates, at progressively farther transverse distances from the central liquid inlet port, is progressively reduced to an extent which is commensurate with the pressure drop experienced as the liquid is directed transversely to the outer extremities of the distribution conduit. As a result, there is preferential channeling of the liquid at the central part of the flow channel from the inlet side to the outlet side thereof, and concommitant under-utilization of the peripheral areas of the filter. When the solids in the central portion have been built up to a point requiring backwashing or draining of the filter, the peripheral areas of the filter still have available capacity to separate solids from the feed liquid.
Such transverse maldistribution of the feed liquid in cross-flow filters of the aforementioned type could conceivably be overcome by the provision of header manifolds to introduce feed liquid into the filtration channels at multiple introduction points along the sides of the stacked filter plates, with a corresponding outlet header manifold arrangement at the opposite side of the stacked plates. Unfortunately, however, such provision would significantly increase the overall system pressure drop as well as the complexity of the filter system, since it could be necessary to positively seal the multiplicity of feed liquid branch lines passing from the manifold into the filter.
Another type of stacked plate cross-flow filter which has been commercialized employs a transversely extending liquid distribution conduit with spaced apart openings therein to introduce solids-containing liquid into the flow channel between adjacent stacked plates, but instead of a central inlet port to flow the solids-containing liquid to such conduit, the liquid is axially fed into the conduit from a feed line connected to a transverse extremity of the conduit. Filters of such type are available from Millipore Corporation (Bedford, Mass.) under the trademark Pellicon.RTM.. This feed arrangement results in a progressive diminution of the liquid pressure at increasing transverse distances from the feed end of the distribution conduit, which in turn results in progressively transversely decreased cross-flow rates of liquid in the flow channel.
In an effort to overcome the aforementioned liquid flow maldistribution characteristics of stacked plate filters, filter plates have been constructed with baffle elements defining discrete flow channels, with the intent of achieving a more uniform distribution of the solids-containing influent liquid across the full areal extent of the filter elements in the flow channels of the filter.
A filter plate commercially available from Toyo Soda Manufacturing Company, Ltd. (Tokyo, Japan) features a structure in which solids-containing influent liquid is introduced to the flow channel at a central inlet port at one side of the plate. A wall is disposed in front of the liquid inlet, extending upwardly from the floor of the flow channel and transversely toward the extremities of the flow channel, to divide the influent stream into two outwardly directed streams. Downstream from such stream-splitting wall is a longitudinally extending divider partition, the stream-splitting wall and the divider partition together forming a "T" construction when viewed in plan view. Longitudinally spaced from and parallel to the stream-splitting wall are a series of baffle partitions on either side of the divider partition. The baffles extend transversely part way across the flow channel on either side of the divider partition, so that there is formed a serpentine flow path for each of the split streams, on the respective sides of the partition. A unitary liquid outlet port is provided at the opposite side of the stacked plates from the inlet port, whereby the respective serpentine flows are finally joined and discharged from the flow channels of the filter.
Although the dual serpentine flow path arrangement described above provides a somewhat better distribution of liquid flow across the areal extent of the filter paper element, the sharp turns in the flow path at the extremities of the baffles create edge and entrance effects in the flow streams which produce substantial dead space and bypassing therein. As a result of such anomalous flow phenomena, the filtration efficiency of the baffled serpentine flow arrangement is significantly reduced.
Accordingly, it would be a substantial advance in the art to provide a filter plate characterized by substantially uniform transverse distribution of liquid from a unitary liquid feed port, and highly uniform liquid cross-flow across the full transverse extent of the flow channel formed when plates of such type are stacked to form a cross-flow filter.
It therefore is an object of the present invention to provide a filter plate of such type, which is simple and efficient in construction and operation.
It is another object of the invention to provide a cross-flow filter comprising stacked filter plates of such type.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.